Motivation
1.2 Greenhouse Effect and Impacts on the Climate
6 1 Motivation
1.2.1 Greenhouse Effect
Some gases contained in the atmosphere have a filtering effect: they let the majority of short-wavelength solar radiation pass through, while partly absorbing infra-red radiation emitted from the Earth, leading to a heating-up of the lower layers of the atmosphere. These gases, accounting for this so-called greenhouse effect, are hence termed greenhouse gases. They bring about a natural net warming of about 33◦C.
Without the present composition of the Earth’s atmosphere, a temperature of−18◦C would predominate on Earth. The atmosphere and the oceans balance the heat bud- get and provide for heat exchange between day and night, summer and winter, polar and equatorial zones. Without an Earth-like atmosphere, temperature differences of 250◦C between day and night occur, for example on the Moon’s surface, to draw a comparison (Borsch 1992).
A distinction is made between the natural greenhouse gases and those produced by man, the so-called anthropogenic greenhouse gases. Some of the greenhouse gases are both of natural and anthropogenic origin. Table 1.1 shows the contribution of the various greenhouse gas types to the natural and anthropogenic greenhouse effect. The most significant greenhouse gas is carbon dioxide (CO2). It is produced through energy consumption in the combustion of carbonaceous fossil fuels such as coal, natural gas and crude oil. In the process, dead organic substance becomes oxidised to carbon dioxide, which is given off to the atmosphere. The quantities discharged this way to the atmosphere amount to about 26 thousand million tonnes of carbon dioxide1per year (2005). Added to this, there are further, inexactly quan- tifiable, amounts of emitted carbon dioxide from forest clearing and through soil degradation. The contribution of these emissions is estimated at about 3–7 thousand million tonnes of carbon dioxide per year.
Table 1.1 Present concentrations of greenhouse gases and their contribution to the natural and anthropogenic greenhouse effect (data from IPCC (2007b) and Beising (2006))
Chloro-
Carbon Methane fluorocarbons Nitrous Ozone Water Greenhouse gas dioxide CO2 CH4 CFCs oxide N2O O3 vapour
Concentration: 280 ppm 0.7 ppm 0 270 ppb –2.6%
pre-industrial time (about 1800)
Today (2005) 379 ppm 1.8 ppm 0.5 ppb 319 ppb 25 ppb 2.6%
Increase rate (2005) +1.9 ppm/a +2 ppb/a 0.8 ppb/a
Emissions (2005) 26 Gt/a 400 Mt/a 0.4 Mt/a 15 Mt/a 0.5 Gt/a Contribution to natural
greenhouse effect= temperature rise
26% 2% – 4% 8% 60%
Contribution to anthropogenic greenhouse effect
61% 15% 11% 4% 9% –
1One tonne of carbon corresponds to 3.67 tonnes of carbon dioxide.
1.2 Greenhouse Effect and Impacts on the Climate 7 The CO2 emissions of anthropogenic origin may be low compared with those of natural origin, but then the natural CO2 emissions are counteracted by reactions of decomposition in the same order of magnitude. CO2emissions of 120 thousand million tonnes of carbon per year released through respiration and decay are in turn extracted from the atmosphere by photosynthesis (Heinloth 2003). The atmo- spheric CO2reservoir, which is an essential part of the global carbon cycle, being the base material for the carbon in the biosphere, amounts to about 750 thousand million tonnes of carbon at present. Referring to this reservoir, annual anthro- pogenic CO2 emissions constitute about 1%, half of which remain in the atmo- sphere, the rest mainly dissolving into the oceans. On the whole, CO2 emissions have led to a rise in CO2 concentrations in the atmosphere through the years and hence to an increase of the atmospheric CO2 reservoir. At the moment, the annual increase amounts to about 1.9 ppm. The CO2 concentration reached in 2005 was at about 379 ppm. The CO2 concentration before the industrial revolution (about 1750–1800) has been reconstructed through ice cores sampled in Antarctica and was determined at about 280 ppm (IPCC 2001b, 2007b; Borsch 1992; IPCC 2001a, 2007a).
In addition to CO2, other greenhouse gases are discharged into the atmosphere through human activities. This group of gases includes methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons. The impact of the various greenhouse gases in causing the greenhouse effect arises, besides from the emitted quantity, from the residence time of the gases in the atmosphere and their molecular structure which determines the heat absorption capacity. The concentrations of all greenhouse gases are evaluated corresponding to their climatic effect and indicated as CO2equivalent.
In 2005, the sum of all long-lived greenhouse gases was 455 ppm, with CO2making the greatest contribution. About 50% of the anthropogenic greenhouse effect has to be attributed to the energy sector (inclusive of the entire transportation sector; 80%
of this fraction is caused by CO2).
In order to determine the effect of natural or anthropogenic factors on the radia- tive balance in the atmosphere, the current assessment reports of the IPCC apply the concept of radiative forcing. It indicates the change of the net irradiance out of solar irradiance and terrestrial radiation. Figure 1.8 shows the change of radiative forcing due to anthropogenic greenhouse gases and aerosols and the changes in solar irradiance and in land use for the period from 1750 to 2005. It can be noticed that the long-lived greenhouse gases involve a marked increase of the radiation flux, with the impact of CO2of more than 1.5 W/m2 dominating. The contributions of the other factors to radiative forcing are significantly smaller, with both negative and positive impacts being implied.
It should be taken into consideration, though, that the scientific state of knowl- edge about radiative forcing is very heterogeneous in regard to the individual fields.
Only in the case of the greenhouse gases is the level of knowledge high; concerning the effect of the aerosols and other substances, the level is low or very low.
The greenhouse effect induced by human activity through the intensified emis- sion of climate-relevant trace gases is held, for the predominant part, responsible for the rise of the temperature by 0.74◦C in the past 100 years (IPCC 2007b).
8 1 Motivation
CO2 N2O
Halocarbons Tropospheric Stratospheric
(-0,05)
Land use Black carbon on snow
(0,01) Long-lived
greenhouse gases
Ozone Stratospheric water vapour
Direct effect Total Aerosol
Cloud albedo effect Linear contrails Solar irradiance Total net human activities Surface albedo
CH4
Human activities Natural process
Radiative forcing of climate between 1750 and 2005
−2 −1 0 1 2
Radiative forcing (watts per square metre) Radiative Forcing Terms
Fig. 1.8 Change in radiative forcing in the period 1750–2005 (IPCC 2007b)
1.2.2 Impacts
A small temperature increase of even few degrees can lead to a far-reaching change of the global climate. A warming process will shift the climatic zones. The subtrop- ical dry zones, for example, will expand poleward into the currently fertile regions in southern Europe, the USA, China, South America and Australia. On top of this, climatic variations and climate extremes like storms, hurricanes, storm tides, periods of drought and heavy rains will become more frequent and stronger. The sea level will rise because of melting ice masses on land and through the expansion of water, thus threatening coastal regions. In what way and to what extent plants and animals are capable of adapting to the climate change depend on the rate at which the climate alters (Heinloth 2003).
1.2.3 Scenarios of the World Climate
The IPCC’s assessment reports provide a comprehensive presentation of the current standard of knowledge in climate modelling (IPCC 2001b, a, 2007b, a). The task of climate modelling is to determine the climate system’s reactions to natural or
1.2 Greenhouse Effect and Impacts on the Climate 9
B1 A1T B2 IS92a A1B A2 A1FI
Several models all SRES Models ensemble
envelope IS92a
B2 B1 A2 A1T A1B
A1FI Model average
all SRES envelope
Several models all SRES envelope
All SRES envelope including land -ice
uncertainly B2
B1 A2 A1FI A1T A1B
2000 2020 2040 2060 2080 2100 5
10 15 20 25 30
(a) CO2 emissions
Year
B1 B2 A1T IS92a A1B A2 A1FI
2000 2020 2040 2060 2080 2100 300
500 700 900 1100 1300
(b) CO2 concentrations
Year
Several models all SRES Models ensemble
all SRES envelope IS92a
B2 B1 A2 A1T A1B A1FI
2000 2020 2040 2060 2080 2100 0
1 2 3 4 5 6
Temperature change (°C)
(c) Temperature change
Year
Model average all SRES envelope
Several models all SRES envelope
All SRES envelope including land -ice
uncertainly B2
B1 A2 A1FI A1T A1B
2000 2020 2040 2060 2080 2100 0.0
0.2 0.4 0.6 0.8 1.0
(d) Sea level rise
Year
Sea level rise (metres)
CO2 emissions (GT C/yr) CO2 concentration (ppm)
envelope
Fig. 1.9 Scenarios of the global CO2 emissions (a), CO2 concentration (b), temperature rise (c) and sea level (d) (IPCC 2001b)
anthropogenic changes, such as the increase of the CO2concentration, and thus the resilience of the system. A summary of the calculations is presented in Fig. 1.9.
Scenarios of the global energy consumption and the associated emissions up to the year 2100 (Special Report on Emission Scenarios (IPCC 2001c) (SRES 2001)) are intended to cover a wide range of possible developments, and they form the basis for the calculation of the world’s climate in the long term. Figure 1.9a shows the CO2
emissions for different scenarios which are used for numerical climate simulations.
Complex climate models are based on the conservation of mass, impulse and energy in a three-dimensional grid encompassing the globe and have to take into account atmosphere, oceans, continental surfaces, the cryosphere, the biosphere and their interactions as individual components. The further development of the partly very simple models is in progress.
The different scenarios of the CO2 emissions assume a rise of the CO2 concen- tration in the atmosphere to values between 540 and 970 ppm up to the year 2100 (see Fig. 1.9b) (IPCC 2001c; SRES 2001). According to the assessment report of 2007, temperature increases of the global mean surface temperature between 2.5 and 4.1◦C by the end of this century in comparison to the mean value between 1961 and 1990 were determined for selected scenarios (see Fig. 1.9c). The source of uncertainty on the one hand lies in uncertainties of the climate model calculations
10 1 Motivation and, on the other, in the wide range of emission scenarios investigated. According to Fig. 1.9d, the average sea level will rise by 21–51 cm; in higher latitudes, though, up to 1 m; in the North Sea, it will rise by 50 cm (IPCC 2007b).
Even if the CO2concentrations were frozen at today’s level (which is tantamount to an almost complete reduction of the CO2emissions worldwide), both the temper- ature and the sea level would continue to rise. This can be put down to the interaction between troposphere and ocean. While the troposphere responds to changes of con- centrations and the associated radiative forcing on a timescale of less than 1 month, the timescales in the case of near-surface sea water range between years to decades, and even centuries in the case of the deep ocean and ice caps. So, even with freezing today’s CO2concentrations, the temperature would still rise by about 0.5–0.6◦C on the whole, with the biggest part of the increase happening within the next 100 years.
These relationships underline the need for a quick and drastic reduction of CO2
emissions, precisely because our climate reacts with great inertia to the increase of greenhouse gases. It also becomes clear, though, that global warming can only be limited, not negated, even by intensive abatement efforts. In the so-called stabilisa- tion scenarios, CO2emission is reduced to achieve a stable equilibrium concentra- tion in the atmosphere.